A gas rich in carbon dioxide contains at least 65% carbon dioxide. It also contains at least one other component chosen from the following list: oxygen, nitrogen, argon, carbon monoxide, hydrogen, nitrogen monoxide, nitrogen dioxide, nitrous oxide, mercury, methanol, ethanol, ammonia or hydrocarbons.
Preferably it contains less than 5% methane, all percentages in this document relating to purities being molar percentages.
The gas rich in carbon dioxide may result from an oxy-fuel combustion, a cement works, a steel works, a steam methane reformer or any other known source.
The present invention proposes, amongst other possibilities, to carry out the final cooling stage of a carbon dioxide purification unit feed gas in a shell-tube exchanger.
The feed gas is cooled in the tubes which are surrounded by a bath of carbon dioxide at its triple point.
There are many advantages associated with operating the system at its triple point.
Firstly the pressure of the shell is perfectly stable at the triple point. If the compressor which compresses the vaporised carbon dioxide coming from the shell tube exchanger removes too much gas, the liquid will flash, forming solid and gas, the whole resting at the triple point pressure. On the other hand, if too little gas is removed, the pressure cannot increase to a substantial extent, due to the presence of frozen carbon dioxide. When the solid phase is in the form of micro-crystals mixed in with the liquid, a liquid-solid “slush” is formed, which increases the solid-liquid exchange surface, as compared with the situation when large blocks of frozen carbon dioxide are formed in the liquid.
Secondly, the feed gas will be cooled as much as possible, to increase the yield of produced carbon dioxide and thereby improve the specific energy and specific cost of the plant (the additional expense to capture the additional tones of carbon dioxide is less than the average expense per tonne).
Thirdly, the feed gas cannot freeze since the cold source is stabilized at the triple point temperature and the feed gas will necessary be above that temperature.
The arrangement allows the solid carbon dioxide to be stored and melted, meaning that the energy can be stored and/ or the liquefaction capacity increased.
In the case where energy is stored, when energy prices are low, an additional compressor (or the product compressor) removes more gaseous carbon dioxide than would naturally be vaporized, to create a suction effect. The liquid in the storage then flashed forming gas (to stabilize the pressure at the triple point) and solid, which mixes with the remaining liquid.
When energy prices increase, the liquid carbon dioxide is melted by sending additional gaseous carbon dioxide into the storage; the solid carbon dioxide melts by liquefying the gaseous carbon dioxide, the additional liquid formed is pumped outside the cold box and the carbon dioxide can be produced at the production pressure, without using the product compressor.
The product compressor is designed to take advantage of these flow variations, the flowrate for a centrifugal compressor being down to 80% of the nominal flowrate. One solution to the problem could be to use three smaller compressors, representing each 50% of the nominal flowrate. In this case, under normal operation, two compressors would operate. During the storage of solid CO2 phase (when energy costs are low), three compressors would operate and during the high energy cost period, one compressor and a pump would operate In this case, the pump is required to pressurize the liquid coming out of the shell and tube heat exchanger prior to mixing it with the remainder of the CO2 product (either liquid or supercritical).
Under normal operation, the solid carbon dioxide formed and builds up in the storage. At peak production, more carbon dioxide is available to be liquefied, it is sent in gaseous form to the storage where it liquefies against the solid carbon dioxide, which melts, thus increasing the maximum liquefying capacity, without increasing the dimensions of the apparatus.